1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method which can be suitably applied to a color facsimile apparatus, a color printer, a color copying machine, and a multifunction apparatus having functions equivalent to those thereof, which form a plurality of reference patches with different densities on an image forming member, detect the colors or densities of the patches, and execute desired gray level correction processing on the basis of the detected color information (hue information or density detection information).
2. Description of the Related Art
Recently, a tandem type color printer, a copying machine, a multifunction apparatus having functions equivalent to those thereof, and the like have been increasingly used. These image forming apparatuses include exposure units, developing devices, photosensitive drums, intermediate transfer belts, and fixing devices for yellow (Y), magenta (M), cyan (C), and black (BK).
For example, the exposure unit for the color Y is designed to draw an electrostatic latent image on the photosensitive drum on the basis of arbitrary image information. The developing device forms a color toner image by adhering Y toner to the electrostatic latent image drawn on the photosensitive drum. The photosensitive drum transfers the toner image onto the intermediate transfer belt. The same processing is performed for the remaining colors M, C, and BK. The color toner images transferred onto the intermediate transfer belt are transferred onto an image recording sheet, and then fixed by the fixing device.
Conventional image forming apparatuses often incorporate an image processing unit with a gray level correction function of performing color matching of the densities of secondary and tertiary colors obtained by combining the respective colors C, M, Y, and K on the basis of the density detection information obtained by a density sensor. This is because an image forming unit is configured to operate on the basis of image information based on a YMCK signal processing system. In this case, gray level correction tables are used in the image processing unit.
FIGS. 1A and 1B are spectral characteristic graphs showing examples of the relationship between the reflectances and the wavelengths of the colors C, M, and Y according to the prior art. Referring to each of FIGS. 1A and 1B, the ordinate represents the reflectances of the three colors C, M, and Y; and the abscissa, the wavelengths of the three colors C, M, and Y. According to the gray level correction function of the image processing unit using gray level correction tables according to the prior art, when a material such as toner or a photosensitive member which is different from the previously used one upon replacement thereof or the like is used, and the three colors C, M, and Y are superimposed on each other, the gray hue tends to vary.
The following is the reason why the gray hue varies when a material such as toner or a photosensitive member which is different from the previously used one is used, and the three colors C, M, and Y are superimposed on each other.
Assume that the graph of FIG. 1A shows the spectral characteristics of the colors C, M, and Y of toner before replacement. When a different material or a different manufacturing method is used for toner in its manufacturing stage, for example, the spectral characteristics of the color M may change as indicated by the solid curve in FIG. 1B.
In such a case, even if image data is corrected to stabilize the lightness (L*) of each of the three colors C, M, and Y in a color solid coordinate (L*-C* coordinate) system, when the colors C., M, and Y are superimposed on each other, the resultant spectral characteristics change as shown in FIG. 1B. In the example shown in FIG. 1B, the reflectance of the color M increases near a wavelength of 430 μm as compared with that in FIG. 1A.
FIG. 2 is a graph showing an example of how the gray hue changes with environmental changes of the image forming system, and is an ab-chromaticity diagram obtained by extracting a two-dimensional C* coordinate system from the L*-C* coordinate system. The ab-chromaticity diagram shown in FIG. 2 is a graph obtained by correcting image data using a one-dimensional gray level correction table, outputting a patch to the intermediate transfer member on the basis of the image data, and measuring the patch with a colorimeter. Referring to FIG. 2, the abscissa represents a saturation a* of red or the like; and the ordinate, a saturation b* of the color Y or the like. Referring to FIG. 2, the solid curve represents the locus of white (W) which represents the upper limit of gray, and the solid curve with circles represents the locus of the color BK which represents the lower limit of gray. As is obvious from this graph, the gray balances differ from each other in the saturation a*-b* plane.
Note that patent reference 1 (refer to Japanese Unexamined Patent Publication No. 2000-338742, FIG. 3 and p. 10 in particular) discloses an image forming apparatus associated with a color printer using the above gray level correction table. According to this image forming apparatus, when an exposure system forms a plurality of test pattern latent images on a photosensitive member with laser beams, a developing device develops the test pattern latent images with toner. The developed toner images are held on an intermediate transfer member. The densities of the secondary and tertiary colors formed by properly combining the respective colors C, M, Y, and K are detected by a density sensor. The density detection information obtained by the density sensors is output to an image processing unit. The image processing unit performs color matching on the basis of the density detection information. This makes it possible to always maintain good color balance.
According to an image forming apparatus using a gray level correction table based on the conventional scheme, when toner, an image forming member, or the like which is a consumable is replaced, image data is corrected by using a one-dimensional (density-lightness) gray level correction table which makes the lightnesses (L*) of the three colors C, M, and Y uniform. For this reason, as indicated by an example of the gray hue change in FIG. 2, the gray hue changes in the saturation a*-b* plane, resulting in different gray balances in the respective three colors C, M, and Y. This hinders color image forming with good gray balance.
The recent years allow the advent of an image forming apparatus which can print out color images with a plurality of types of image structures. An image forming apparatus of this type has, for example, five image structures #1 to #5. With image structure #1, although the resolution is high, it is difficult to realize good highlight tone reproducibility, and moiré tends to occur due to overlapping of the respective colors C, M, Y, and K. With image structure #2, although the overall grayscale characteristics are excellent, the resolution is low, and it is difficult to realize good highlight tone reproducibility, resulting in a lot of moiré. With image structure #3, although the resolution is high and the highlight tone reproducibility is excellent, a lot of moiré is produced. With image structure #4, although the resolution is high and the overall grayscale characteristics are excellent, the image structure produces a sense of noise. With image structure #5, since the resolution is high and toner is transferred in dots, the overall grayscale characteristics are good, and no moiré occurs. However, a sense of noise is left.
The image forming apparatus capable of printing out color images with five image structures #1 to #5 described above has the above merits and demerits concerning the respective image structures, and is designed to calculate a gray level correction table for gamma correction of image data for each image structure. A gray level correction table is calculated for each image structure and is formed dependently on the ease of adjustment concerning each image structure. If, for example, adjustment results on the colors C, M, and Y differ for each image structure, lightnesses are set as follows in correspondence with the respective gray levels:
Color CColor MColor Y(%)(%)(%)CharacteristicsImage606060normalStructure#1Image555560yellowishStructure#2Image656065greenishStructure#3Image656565dark overallStructure#4Image555555light overallStructure#5
In association with a color printer using the above gray level correction table, an image forming apparatus is disclosed in patent reference 2 (refer to Japanese Unexamined Patent Publication No. 08-292616, FIG. 1 and p. 4 in particular).
The image forming apparatus disclosed in patent reference 2 includes a gray level correction data generating unit and is designed to input and set a highlight gray level correction condition from an input unit to the gray level correction data generating unit and generate gray level correction data on the basis of both the input highlight gray level correction condition values and the density values actually measured from reference patch generated on an image carrier. This makes it possible to generate gray level correction data in accordance with gray level variations at a highlight portion over time, thereby correcting the highlight tone reproducibility with high precision.
The image forming apparatus capable of printing out color images with five image structures #1 to #5 in the conventional scheme has merits and demerits concerning the respective image structures. Such merits and demerits occur in different manners depending on the patterns of image data.
An image forming apparatus of this type is configured to select an image structure in accordance with user's needs, and individually performs gray level correction for each image structure. For this reason, there are both adjustable and unadjustable portions. If adjustment results on the colors C, M, and Y differ for each image structure, brightnesses corresponding to the respective gray levels become those described above, the adjustment balance among the colors C, M, and Y deteriorates. As a consequence, the color appearance of gray obtained by superimposing C, M, and Y toner layers becomes yellowish or greenish, resulting in different hues.
Even if, therefore, the technique of the image forming apparatus disclosed in patent reference 2 is used without any change, when the image structure is changed, an output color image may differ in appearance (gray level) from the previous image. This hinders an improvement in image forming quality in a color printer and the like.
In addition, the above conventional gray level correction table is designed to make calculation in accordance with “grayscale-characteristic-oriented”, “gray-level-oriented”, and “reproducibility-oriented” in order to eliminate variations in characteristics between a plurality of image forming apparatuses. Such a table has the following characteristic features.
According to a “grayscale-characteristic-oriented” calculation method, although a gray level correction table can be calculated by making the most of dynamic ranges, different gray balances may occur among the apparatuses. According to the “gray-oriented” calculation method, the ratios between the three colors C, M, and Y are set in advance, and a table is calculated such that the ratios between the respective colors become equal. This can hold the gray levels among apparatuses constant. In one image forming apparatus, however, dynamic ranges that can be originally reproduced may be wasted. In addition, if there is a color with an extremely low dynamic range, since ratios are set with reference to this color, image forming outputs may differ from each other among the apparatuses.
According to the “reproducibility-oriented” calculation method, since a table is generated so as to set densities equal to the preset densities of the three colors C, M, and Y, even if a color with an extremely low dynamic range is produced, it does not become too light. If, however, the dynamic range is low, a table for a portion with a high density is always set to the maximum value. This results in poor grayscale characteristics of a printout and may not guarantee gray balance in this region.
An image forming apparatus which handles a gray level correction table of this type and color balance is disclosed in patent reference 3 (refer to Japanese Unexamined Patent Publication No. 2002-287271, and FIGS. 2 and 3 in particular). This image forming apparatus detects a temperature and humidity in the apparatus and the time during which a recording material is exposed to air when performing exposure processing with respect to the recording material, and corrects an exposure amount for the recording material so as to cancel out the influences imposed on image forming operation on the basis of the detection information. This makes it possible to prevent variations in color density and color balance in visible images after image forming processing.
A method of setting gray balance is disclosed in patent reference 4 (refer to Japanese Unexamined Patent Publication No. 9-284577, and FIGS. 1 and p. 3 in particular). According to this gray balance setting method, a reference chart is generated by a reference output apparatus whose gray balance is adjusted in advance, and the characteristic data of the reference output apparatus is acquired from the measurement value based on this reference chart, thereby adjusting the gray balance of the target output apparatus by using the characteristic data. This makes it possible to set gray balances with high precision in a plurality of target output apparatuses.
A calibration method in a color image forming apparatus is disclosed in patent reference 5 (refer to Japanese Unexamined Patent Publication No. 2001-144982, and FIGS. 2 and p. 3 in particular). According to this calibration method, the density of each patch of the test chart output from the image forming apparatus is compared with the density of a reference patch of a reference chart prepared in advance in conjunction with one of the three colors C, M, and Y or the color BK, and density adjustment is performed by using density data on the test chart that matches the reference chart. With regard to the remaining colors of the three colors C, M, and Y, density adjustment is performed on the basis of the density data of gray patches selected from gray balance charts. This makes it possible to comprehensively determine adjustment values from a plurality of results concerning gray balances.
According to the image forming apparatus which handles gray level correction tables based on the conventional scheme and color balances and the image forming apparatus in patent reference 3, however, in order to eliminate variations in characteristics between image forming apparatuses, “grayscale-characteristic-oriented”, “gray-oriented”, and “reproducibility-oriented” calculation methods are often determined in the design stage. This is based on the premise that the densities of the three colors C, M, and Y do not change over time.
For this reason, in the conventional scheme, according to the present circumstances, when the densities of the three colors C, M, and Y change over time, the user cannot arbitrarily select an intended one of the “grayscale-characteristic-oriented”, “gray-oriented”, and “reproducibility-oriented” calculation methods. When, therefore, the densities of the three colors C, M, and Y change over time, even if the gray balance setting method in patent reference 4 or the calibration method in patent reference 5 can be used, the inability of selecting the “grayscale-characteristic-oriented”, “gray-oriented”, or “reproducibility-oriented” calculation method hinders a stable color image from being printed out.